October 23, 2016 - 

I know I'm not the only who's noticed, and neither are you.  The move of corporate Hollywood from the story-encapsulating images of 70's posters, or 50's monsters carrying the girl, has now given way in 00's-10's Hollywood to a sort of generic portrait gallery, with the same pose meant to "ritually" identify each particular sub-genre.
It's already become a favorite viral joke among Internet wags on Tumblr and Pinterest, to put together Genre-galleries by poster image--Like the "Eye is the Window of the Soullessness" scifi/horror movie, or the "Back-to-Back 'Pretty Woman' Dueling Rom-Com-Couple":

It's not that complicated an idea--Studios, after all, have a jealous habit of wishing they were Somebody Else's Movie You Liked Better, and dressing up like their favorite role models.

But this blog isn't here to do viral jokes.  We're on a proactive mission.  Anyone can viral-gag asking a question, the hard part is in trying to answer them.  If we know the cause of the disease, we're that one step closer to finding the cure.  (If any.)

If you want to find some root cause of the disease, think back to the exchange between Geoffrey Rush and Tom Wilkinson in Shakespeare in Love:
"But I have to pay my actors and author!"
"Share of the profits."
"There's never any." 
"Of course not!"
"I think you may have hit upon something..."
Actors today are a little smarter than they were in Shakespeare's time.  After the case of Art Buchwald v. Paramount Pictures, suing for his stolen story on 1988's "Coming to America" and winning, discovered that the #3 box-office smash of 1988 "hadn't turned a net profit" under Paramount's "Hollywood Accounting", and thus had nothing to pay out, actors know they're not stupid enough to ask their agents for a share of movie profits anymore.
What they now want are Points--Points that will establish their "sales ability" as recognizable studio stars, share in the investment, a legal co-producer credit if they do share in the production, a profit off of any marketing of their character outside of the movie, and negotiable value as a Hollywood power-player staple that will allow them to leverage more demands on their next movie contract.

Among other complicated studio entanglements, this now means two things:
1) Actors are chiefly interested in marketing their faces for a living, and
2) Marketing those Faces For Sale now comes at a PRICE.  Talk to their agents, or face the consequences.
Studios, however, are interested in only one aspect of marketing, apart from what agents demand:
1) Audiences already know the movie they're buying a ticket for, walking in.  You're selling them the intimate in-knowledge of an old friend who's come back again, not some total-stranger movie out of nowhere they, like, wouldn't know.

Sometimes, the actor has negotiated his character to be the entire selling point of the marketing.  
Often this can be achieved with just an arresting ultra-closeup of nothing but the actor's face, and the teasing word-economic story ambiguity of the Vertical Tagline:
The teaser poster for the 2013 Carrie remake showed Chloe Moretz's face, with the vertical tagline "You. Will. Know. Her. Name."  The teaser poster for 2016's Jason Bourne showed Matt Damon's face, with the vertical "You. Know. His. Name."  

Well, now that we're all familiar on a name basis, let's have some drinks; who's for a game of Pictionary?

But this brings up the obvious question:  How can you tease an action movie, which is 90% ABOUT the lead actor/character hero, and not contractually show his face for another month or two?
Simple:  He's there, but with his back to us.  He's too busy Facing His Lone Destiny.

Or just carries some instantly recognizable weapon in his hand.
Don't worry though, the hit franchise character is still iconic to his fans even from the back or other isolated body parts, so You'll Know His Name.

But the most crucial Job One of a franchise sequel is to sell not a hand or nape of a neck, but to sell the LOGO--Like the logos of opera masks and French orphans that now ride atop every NYC taxi on Broadway, a shaped symbol logo must singlehandedly conjure up an entertainment movie-studio franchise.  
And with six months to a year to wait for the "You Know Its Logo" sequel to the earlier hit movie, the most arresting image is that the abstract presence of a logo you thought was gone is now beginning to reappear (ooo!), and won't fully take shape until next May, June or November.  (The one thing a teaser poster must sell is not the title or tagline, but the DATE--One look at the logo, and You Know Its Title.)  As with the damage to Starfleet Headquarters in 2013's "Star Trek: Into Darkness": 
And as usual, Chris Pine--or is it Benedict Cumberbatch?--has his back to us.  Not that we would really be able to see his tiny face next to all that big wreckage even if he was allowed to turn around and show it to us from the front.

But once the title is teased, taken hold, and started production, presumably all the major actors have their contracts ironed out, and are now free to show their face.  We have the second problem--WHOSE face gets to be shown?  Who is the true "star" of the movie?  Ask an ensemble movie of several lead characters, and you may find each actor had an agent who told them they were.  And no studio wants to start that argument.
This brings up that Points problem of "Character marketing", as even a major supporting character who was contractually cast now has to assert his presence in the movie.  Standup comic Robert Wuhl once joked about the system after appearing in 1989's Batman:  "I get a percentage of the marketing of my character--I play the wisecracking reporter who disappears halfway through the movie and doesn't get the girl; I'm hoping for those action-figure sales."
Studios solve that problem with something that allows them even more "Lobby space" for promotion--A poster for each character.  (A practice that started during the early comic-book movies, like the '99 Star Wars: Episode I or '00 X-Men, when anal-retentive fans couldn't wait to judge their first peek at what each new character would look like.)  

To take a harmless example, here are upcoming posters for a 2017 Power Rangers reboot--First we have nameless silhouettes, an abstract tagline against a still-abstract lightning-bolt-logo sky, and no title but a date:

Now, five similar posters on a theme, each with the clear, negotiated face of one of the marketable characters in costume/gear, also with no title but the logo, date and cultural-nudge tagline--Which one are you rooting for?  Red? Gold? Pink?  (I don't care, I never watched the show as a kid.)

Yes, like those old third-grade school days...everyone's special enough to get a poster for participating, and no one gets left out.

And, of course, when a movie does finally open with its final marketable sales image, title and cast/credit list, it doesn't have six posters.  It has one.  With a lot of people who were legally told they would be on it.

To understand this concept, you have to understand about medieval art--In the old days, before Italian Renaissance painters like Brunalesschi developed the 3-D realism of perspective, "flat" 2-D medieval religious art had to determine the size of the figures by who was more important in the hierarchy to the scene depicted.  The saint would always be more ginormous than the almost equally big faithful king/noble patron featured, who was bigger than the normal-sized priests and teeny peasants, and so on, in terms of relative flattery.
This brings us, in our modern days of feudal Hollywood, to the final poster, or "Head salad"--It's the studio's last chance before opening to give every actor the poster-representation they negotiated for, and like the class picture, EVERYBODY has to be in shot.  Just how big we see them in the shot, however, depends on their negotiable role in the movie.

So, as we said at the beginning, it's really not that complicated.  All major-studio movie poster campaigns today boil down to one simple formula--Nobody, Each & Everybody.
Let's take an example from last summer:  "X-Men: Apocalypse", Nobody, Each & Everybody.

Take a look at the Everybody final-poster graduation shot:  James McAvoy/Xavier and Michael Fassbender/Magneto, they're the A-list focus of the story, they're front, center and in your face.  The new breakout supporting-character actors?--You can just spot them somewhere in back, they didn't get as much screen time.  (Insert Sesame-Street Grover voice:  "Near.....Fa-a-a-ar!").  The returning actors from the earlier film?  Don't worry, they made sure they're big enough to make out their faces clearly somewhere in the middle behind the front, or they wouldn't have come back for the sequel.
The villain?  Well, he's this entry's plot, he always has to be looming in back before he can show his face, like Donald Trump looming behind Hillary Clinton talking about healthcare.

Now let's try it with a big upcoming movie we haven't seen yet...Umm, I know:  "Rogue One: A Star Wars Story"!
If the theory holds, we should have the teasingly abstract suggestion of a cool recognized pop-image/logo with no discernible actors and a date, a themed closeup of each supporting character as uniquely market-identifiable dramatic partner of the whole, and one graduation-shot where we get to see Who's Bigger Than Who.  Nobody, Each & Everybody.

There you have it--Told you it was simple.   The answer to the Internet question, "Why do all movie posters look exactly alike?":  Because the studios have no choice.
Because Hollywood Accounting, corporate franchising and agent negotiation have forced them to market every movie exactly alike.  Well...duh.